Biodiesel is the name for a variety of ester-based oxygenated fuels made from vegetable oils, fats, greases, or other sources of triglycerides. It is a nontoxic and biodegradable substitute and supplement for petroleum diesel. Even in blends as low as 20% biodiesel to 80% petroleum diesel (B20), biodiesel can substantially reduce the emission levels and toxicity of diesel exhaust. Biodiesel has been designated as an alternative fuel by the United States Department of Energy and the United States Department of Transportation, and is registered with the United States Environmental Protection Agency as a fuel and fuel additive. It can be used in any diesel engine, without the need for mechanical alterations, and is compatible with existing petroleum distribution infrastructure.
As reported in “Biodiesel: On the Road to Fueling the Future,” (National Biodiesel Board 2001), the disclosure and subject matter of which is hereby incorporated by reference in its entirety, most biodiesel is produced by the process of acid or base catalyzed transesterification. The transesterification process is a low temperature (150° F.), low pressure (20 psi) reaction having a high conversion factor (e.g. 98%) with minimal side reactions and reaction time. The general process is shown in FIG. 1, which is based upon a similar figure shown on page 11 of the above-cited National Biodiesel Board article. A fat or oil is reacted with an alcohol (such as methanol or ethanol) in the presence of a catalyst to produce glycerin and alkyl esters, the latter of which comprises biodiesel. The alcohol is charged in an excess stoichiometric amount to drive the reaction and is recovered for reuse. The catalyst is typically sodium or potassium hydroxide which is mixed with the alcohol prior to the transesterification reaction. The biodiesel is separated from the glycerin. Variations, improvements, and modifications of this general process are described in several patents, including U.S. Pat. No. 5,424,467 entitled “Method for Purifying Alcohol Esters,” issued to Bam et al. on Jun. 13, 1995, and U.S. Pat. No. 6,174,501 entitled “System and Process for Producing Biodiesel Fuel with Reduced Viscosity and a Cloud Point Below Thirty-two (32) Degrees Fahrenheit,” issued to Noureddini on Jan. 16, 2001, the disclosures and subject matters of which are hereby incorporated by reference in their entireties.
Conventional biodiesel production systems are based upon large, fixed base plants which require expensive capitalization and on site construction. For example, in order to generate an economically viable amount of biodiesel product, a conventional biodiesel plant contains large, batch-type reactors, large separation units (e.g., decanters, centrifuges, clarifiers), and distillation columns as tall as 50 to 200 feet or more. As a result, current biodiesel production is limited to discrete locations where fixed plants may be constructed. This results in inefficiencies that may otherwise be obtained by locating a plant near a source of raw materials, or near an end user of the biodiesel product. Further, the conventional process relies upon batch processing, in which the transesterification reaction proceeds in at least two multi-hour stages and in which the separation processes are not able to be performed continuously.